Recently, a system to supply electric power wirelessly has been widely considered. There are two major systems to realize wireless power feeding. One of these systems is an electromagnetic induction system which has been widely known. In the electromagnetic induction system, a degree of coupling between a power transmitting side and a power receiving side is very high and power feeding can be performed at high efficiency. The other system is a magnetic field resonance system. In the magnetic field resonance system, by using a resonance phenomenon positively, it is not necessary for a power feeding source and a power fed destination to share much magnetic flux. There is a common problem between these two systems, which is a safety problem due to heat generated by a foreign substance. Each of these systems uses a magnetic field for power feeding, whereby when a foreign substance of a conductor (such as metal) enters into the magnetic field, an eddy current may flow through the foreign substance and the foreign substance may generate heat.
Thus, a method to detect whether there is a foreign substance from change in an electric characteristic value of an apparatus has been proposed. With such a method, a design restriction or the like is not necessary, and thus, it is possible to detect a foreign substance at a low cost. Specifically, a power feeding system to measure power difference between transmitted power and received power as an electric characteristic value, and to detect a foreign substance from change in the power difference, has been proposed (see, for example Patent Literature 1). Also, a power feeding system to detect a foreign substance by measuring a quality factor, so-called a Q factor, as an electric characteristic value and by comparing the measured Q factor and a threshold (see, for example, Patent Literature 2).